Patterned superhydrophilic-superhydrophobic (SHL - SHB) surfaces have shown promise in droplet-based biochemical assays. However, fundamental understanding of the behavior of liquid droplets on such patterned surfaces has not received much attention. Here, we report wetting dynamics and stability of an aqueous droplet placed over a superhydrophilic spot (θ~0°) surrounded by a superhydrophobic surface (θ~160°). We study the shape evolution (contact angle (θ) and contact line diameter (dc)) of an aqueous droplet placed over a horizontal SHL - SHB surface with its volume (Vd), using experiments and analytical modeling. The results showed that depending upon the Bond number (Bo) and spot diameter (ds), three different regimes: spherical cap with fixed dc and varying θ (Regime I), oblate spheroid with fixed dc and varying θ (Regime II), and oblate spheroid with varying dc and fixed θ (Regime III), are observed. The transition from Regime I to Regime II occurs for Bo~1 whereas that from Regime II to Regime III occurs at Bocr~0.33ds1.30. Analysis of the present case wherein the contact line lies at the boundary of SHL - SHB surfaces, revealed anomaly with respect to the statements of Wenzel, Cassie-Baxter and McCarthy. Further, the stability of a droplet placed over the superhydrophilic spot on an SHL - SHB angular surface is studied using experiments and analytical modeling, which showed that the competition between contact line pinning force (Fp) and gravitational force (Fg) governs its stability. The stable and unstable regimes are identified based on the Bond number (Bo) and spot diameter (ds) and the critical Bond number for stable - unstable transition depends on spot diameter as Bocr~0.5ds-0.93.

中文翻译：

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了解水滴在超疏水表面包围的超亲水点上的润湿动力学和稳定性。

图案化的超亲水-超疏水（SHL-SHB）表面在基于液滴的生化分析中显示出了希望。然而，对液滴在这种图案化的表面上的行为的基本了解还没有引起足够的重视。在这里，我们报道了由超疏水表面（θ〜160°）包围的超亲水点（θ〜0°）上的水滴的润湿动力学和稳定性。我们使用实验和分析模型研究了放置在水平SHL-SHB表面及其体积（Vd）上的水滴的形状演变（接触角（θ）和接触线直径（dc））。结果表明，取决于键数（Bo）和点直径（ds），三种不同的状态：具有固定dc和变化θ的球形帽（区域I），具有固定dc和θ变化的扁球体（区域II），观察到具有变化的dc和固定θ的扁球体（区域III）。从体制I到体制II的过渡发生在Bo〜1，而从体制II到体制III的过渡发生在Bocr〜0.33ds1.30。对接触线位于SHL-SHB表面边界处的当前情况进行分析，发现与Wenzel，Cassie-Baxter和McCarthy的陈述有关。此外，使用实验和分析模型研究了放置在SHL-SHB角表面上的超亲水点上的液滴的稳定性，这表明接触线钉扎力（Fp）和重力（Fg）之间的竞争决定了其稳定性。